1. Field of Invention
This invention relates to testing the continuity of electrical conductors without any physical contact. In particular, this invention relates to a system of using a flood gun on one side of a body to be tested to charge conductors to a given potential together with a scanning electron beam on the opposite side to scan the conductors and detect secondary electron emission from those conductors having electrical continuity between front and back side. The system is employed to test multi-layer ceramic (MLC) laminates prior to firing, sublaminates, and greensheet material.
2. Prior Art
Cost and yield considerations in the manufacture of integrated circuit semiconductor devices mandates the definition of a system to test for defects at an early point in the manufacturing process. Due to the extreme miniaturization in the size of these devices, and the conductors used, conventional electrical testing techniques employing contactors at conductor ends has become increasingly difficult. The high density of conductor ends, uneven leveling and their close proximity to each other are all factors contributing to this difficulty. Moreover, such testing conventionally occurs at a point, generally after firing, in the manufacturing process where significant costs such as heat processing have already occurred. Discarding a module at that manufacturing point is not cost effective. Additionally with the introduction of multi-layer packaging, MLC devices, conventional optical tests can no longer be performed to determine the continuity of conductor lines. Such optical techniques are limited by resolution and a general inability to detect certain types of line and microsoket (via) defects.
The difficulties in testing these devices is compounded by the inherent nature of the conductive material. The conductive material deposited or unfired ceramic substrates (greensheets) has a paste-like consistency posing the risk that physical contact destroys the specimen during testing by introducing surface defects not previously disposed in the material. Accordingly, visual, as well as automatic inspection of a single greensheet has been used to detect surface defects in the greensheet material. These defects include erroneously punched or missing via-holes, incompletely filled via holes and defects in screened connection lines. Such testing is not effective to determine electrical defects such as shorts and opens.
Individual greensheets are subsequently pressed together to form laminates. Defects can occur at this point in the manufacturing process such as opens in the via-connections, opens in the horizontal line connections and shorts between lines. Following lamination, the greensheets are fired in kilns to produce a multi-layer ceramic MLC substrate.
Given these processing steps, testing subsequent to firing, when the material is more easily handled, results in discarding defective modules at a point in the manufacturing process that is not cost effective. In contrast, testing prior to firing, while preferred on a cost basis, has been generally unsatisfactory due to the nature of the conductive material, a paste, and also the deficiencies in existing testing technology.
In order to avoid these difficulties, electron beam testing has been proposed. The prior art is replete with a number of electron beam techniques for electrical continuity testing of semiconductor devices. U.S. Pat. No. 3,373,353 relates to the testing of sheets of dielectric material and particularly to the achievement of local measurements of thickness for detecting thickness defects. A low energy scanning electron beam charges the surface to cathode potential in a manner compatible with photoconductive target charging in a vidicon camera tube. The current across the dielectric layer to its conductive backing is measured with an electronic potential directly applied to a conductive backing on the substrate thereby generating a potential difference across the dielectric. Hence, surface contact at the conductive backing portion is essential.
U.S. Pat. Nos. 3,763,425 and 3,765,898 both relate to non-contact continuity testing of conductors utilizing electron beams. Both measure the resistance of connecting conductors on or embedded in an insulating matrix. A pair of individually controlled electron beams are used which must simultaneously address both ends of the conductor under test. In both patents, special masks are employed, individually tailored to each configuration of conductors of the specimen under test. As shown, for example, in FIG. 3 of the '898 patent, the mask may be a complex structure making loading and unloading of specimens difficult, thereby inhibiting thruput in manufacture. In both patents, the masks stabilize the potential on the specimen surface and act as collecting and measuring electrodes. In the case of the '425 patent, the masks are used to generate secondary electrodes for excitation of the target. Optimization of the operating parameters of the system can be attained but, however, with a penalty of interference in the changing of specimens due to the use of such masks. Other systems utilizing mask techniques include U.S. Pat. Nos. 3,678,384 and 4,164,658.
The prior art also includes a number of proposals to use electron beam techniques in diagnostic analysis of electronic circuits. Some techniques are contactless and others are not. U.S. Pat. No. 4,139,774 relates to an electron beam apparatus that eliminates specimen staining which is caused by contamination in vacuum pumps. The system is designed for specimen surface analysis and not electrical testing. U.S. Pat. No. 4,172,228 utilizes a scanning electron microscope SEM to irradiate selected areas of an integrated circuit until failure occurs. U.S. Pat. No. 4,169,244 relates specifically to electron probes for testing electronic networks. The system requires electrical stimulation of the unit under test by means of external electronics.
I.B.M. Technical Disclosure Bulletin, Vol. 12, No. 7, December, 1969 discloses in very general terms the use of two separately controlled but simultaneously active scanning electron beams. The system is therefore similar to that disclosed in U.S. Pat. Nos. 3,763,425 and 3,764,898. The beams are focused at two distinct points in the array and the potential which exists at one energizing point is measured by capturing scattered secondary electrons with a pickup and measuring device.
I.B.M. Technical Disclosure Bulletin, Vol. 23, No. 5, October 1980, discloses a system that generates a voltage contrast at test points of a specimen utilizing a scanning Auger microprobe (SAM) or a scanning electron microscope (SEM) by biasing the specimen. The testing of IC chips occurs where the biasing corresponds to binary zero and one logic levels. Although the system is contactless and utilizes a commercially available electron beam instrument, it is not suited for testing large area specimens having a dielectric matrix or when physical electrical connections to the specimen are not present. Another SEM technique for testing IC chips is disclosed in I.B.M. Technical Disclosure Bulletin, Vol. 23, No. 7A, December 1980. The system is not contactless, utilizing multiple connections to the chips on a module to drive them. The system is therefore not suitable for soft uncured multi-layer ceramic materials.
Accordingly, within the known technology of testing, multi-layer ceramic laminates, sublaminates and greensheets, a requirement still exists to define a system to determine the conductive nature of an uncured specimen without actually contacting the device. Additionally, testing should take place at a rate commensurate with other processing steps to enhance thruput. Testing should also take place at a point in the manufacturing process where defective modules may be discarded prior to subjecting them to significant manufacturing costs.